Marsha Schuchard has found that grail of researchers - original documents that confirm suspicions about her subject. In this case they are surviving records of the unworldly Moravian Chapel in Fetter Lane showing how William Blake's family were worshippers at this shrine of eroticism.

The Moravians raised prayers to the side-hole of Jesus, the spear wound which was depicted with a vulval shape and was the subject of hymns and ecstasies. "At the love feast our Saviour was pleased to make me Suck his wounds," the woman here identified as Blake's mother wrote in the documents Schuchard has uncovered.

There was clearly a certain amount of hanky-panky with the spirits during the turbulent period in the mid-18th century which was called the "Sifting Time" in Moravian history, though some of the revels ("one brother breaking wind over another's tea-cup") seem more like adolescent buffoonery than manifestations of sexual love.

The head of the chapel, Count Zinzendorf, had a pioneering policy of sex-education for young marrieds, including texts to recite during climaxes, such as "when my dear husband lets his oil sizzle in me, this grace is a sacrament". Zinzendorf argued that the Old Testament commandment against adultery was out of date because it was based on a time when polygamy was common. He himself had an open relationship with a 14-year-old girl whom he made an eldress of the church.

In a chain of association rather than reasoning, Schuchard shows how this influenced Blake in his relationship with Catherine, an illiterate servant girl whom he married in 1782. Blake said "there should be a community of women", meaning that women should be held in common. This proving too ambitious, he suggested getting a concubine. Mrs Blake thought otherwise, hence the tears of the title.

Blake, Schuchard says, learned such manoeuvres as withholding orgasm in order to retain seminal fluid to nourish the brain, putting his wife under the pressure of "enabling her husband to achieve the prolonged erection necessary to the visionary process". It is clearly supposition that Mrs Blake was so beleaguered, though a man who wrote "The lust of the goat is the bounty of God" doesn't seem the sort to compromise on his conjugals.

Schuchard also traces Blake's journey through the mystical underground of the supposedly "enlightened" 18th century, a world rich in knowledge of the Jewish Kabbalah, Muslim sensuality, Tantric yogis and Chinese love-texts. She shows an (arguable) association of some of Blake's early work with people on the sexual scene of the day, including the quack Dr James Graham, with his virility-enhancing "celestial bed", and the mesmerists, who were accused of erotic titillation in the guise of therapy. It adds up to a fantastic miscellany of sex and mysticism, though sometimes Schuchard seems to be working hard to make the pieces of her jigsaw fit.

She analyses the poet's extensive prophetic books, which only an adept can find anything but tedious, and gives a close reading of the more rewarding poems such as "The Crystal Cabinet" and "Long John Brown and Little Mary Bell". These, she says, suggest sex practices aimed at visionary breakthrough, while Swedenborg's belief that "the great toe communicates with the genitals" is demonstrated by Blake's self-portrait "William", showing his body flung back while a flaming star descends toward his left foot. Weird stuff, indeed.